This heat wave isn’t just stifling — it’s deadly.

Extreme temperatures continued Thursday across a large swath of the country, killing more than a dozen people, at least two police dogs and likely contributing to the death of Franklin the rhinoceros at a Mississippi zoo. Arkansas fire departments were volunteering to hose down overheated cattle, and people as far north as Maine were trying to stay cool.

High school football teams and marching bands practiced indoors or canceled altogether. Tennessee election officials touted air-conditioned polling places as a way to bring in voters, and many cities set up cooling centers for those who needed a break from the sun.

The Northeast, including Maine, is on pace for a record-setting summer as the hot and muggy weather pattern persists.

In Portland, which has had nine straight months of above-normal temperatures, Anthony Alfreds likes to cool down by leaping off a pier into the cold harbor waters.

“It sure does beat the heat,” the 13-year-old shouted this week while giving two thumbs-up after cannonballing into Portland Harbor on a steamy, 91-degree day.

It isn’t the one-day highs that are catching the attention of meteorologists in the Northeast; it’s the prolonged heat, with day after day of high temperatures and humidity.

July also was the ninth straight month of above-normal temperatures in Caribou. Boston’s temperatures have been above normal every month since January, and Baltimore, Philadelphia and New York have been warmer than usual since March.

The streak is likely to continue through the summer and into early fall, said Mark Wysocki, a meteorologist and professor at Cornell University in Ithaca, N.Y. The weather pattern — with the jet stream farther north than usual — isn’t showing signs of breaking up anytime soon, he said.

Many locations in the Northeast had the warmest meteorological spring on record for March, April and May. It wouldn’t surprise Wysocki if those same places set records for the summer, as well.

“Not only the summer, but also for the year, 2010,” he said.

In Portland, the string of monthly above-normal temperatures goes back to November and includes the warmest March and April on record and the second-warmest May and July.

In the South, the scorching temperatures and high humidity made it feel like at least 100 degrees in many places, with heat advisories in effect for 18 states.

“This heat wears on everybody,” said Sandy Shamburger, who runs Rankin Sod Farm in Brandon, Miss. “We rigged up lights on a sod harvester so we can work at night.”

Still, not even nightfall brings much relief, with temperatures sometimes lingering in the 80s overnight.

In Columbia, S.C., 33-year-old Kylin Doster tried to stay under the shade of his umbrella stand as he hustled to serve up steaming hot dogs. He said he works from 10 a.m. to 3 p.m. on Main Street, then sets up outside a biker bar north of town from 11 p.m. to 3 a.m.

“It’s really, really hot now, but it even stays hot at night,” Doster said. “It don’t make any difference. It just stays hot.”

The heat has been blamed for at least 16 deaths in Mississippi and Tennessee alone, including a man who had a heart attack while mowing his lawn and a construction worker who was spreading concrete. Maryland authorities on Thursday reported two heat-related deaths from early last week.

Two concrete sections of U.S. Highway 49 in central Mississippi buckled Tuesday, when temperatures hit 103 degrees.

“I can assure you, it was probably 120 degrees on the concrete,” said Steve Grantham, assistant district engineer for the Mississippi Department of Transportation.

In the nation’s capital, temperatures have hit at least 90 degrees on 45 days so far, said National Weather Service meteorologist Heath Sheffield. There were only 22 such days last year.

In South Portland, John Chadbourne, 35, has been seeking relief by going to a water park, cranking the air conditioner up in his home and car, or coming to the Maine State Pier.

“And we’ve spent about $700 on the ice cream truck,” he said, smiling at his 7-year-old son fishing nearby.

In its three-month outlook, the National Weather Service Climate Prediction Center is calling for above-normal temperatures to continue across the Northeast.

A weather pattern is forming in the Pacific Ocean that suggests the weather could return to normal during the fall and into the winter, Wysocki said.

In the meantime, people shouldn’t expect the warm-weather streak to continue forever, said George Jacobson, Maine’s state climatologist and a professor at the University of Maine’s Climate Change Center.

“About a year and a half ago we had the coldest day ever in Maine,” Jacobson said. “On Jan. 16, 2009, it was minus 50 in Aroostook County.”